1. Field of the Invention
The present invention relates to a digital TV(DTV), and more particularly, to a vestigial sideband (VSB) demodulating device and method in a DTV receiver, in which a VSB signal is received for independent carrier wave recovery and symbol clock recovery.
2. Discussion of the Related Art
Generally, a VSB mode of Grand Alliance employed as the standard of a digital TV (for example, HDTV) transmission mode in the United States of America and Korea modulates one of two side bands generated up and down based on a carrier wave when amplitude of a signal is modulated while attenuates the other. That is to say, the VSB mode transmits one side band spectrum of a base band to a pass band, so that a band region can effectively be used.
In case of VSB modulation, DC spectrum of a base band is changed to a tone spectrum in a pass band. The tone spectrum is generally called a pilot signal. That is, a broadcasting station modulates a VSB signal, a receiver blows the pilot signal away in the air to exactly demodulate the signal.
FIG. 1 is a block diagram illustrating a general digital TV receiver. Referring to FIG. 1, a VSB demodulating device includes an analog-to-digital (A/D) converter 103, a phase divider 104, a multiplier 105, a timing recovery portion 108, a carrier waver recovery portion 113, and a channel equalizer 107.
That is, if a radio frequency (RF) signal modulated in a VSB mode is received through an antenna, a tuner 101 selects a desired channel frequency. Then, the tuner 101 transits a VSB signal of an RF band inserted in the channel frequency to a fixed intermediate frequency (IF) band of 44 MHz or 43.75 MHz and properly filters other channel signal.
An output signal of the tuner 101 passes through a surface acoustic wave (SAW) filter 102 employed to remove other band signal and a noise signal and used as an analog matched filter.
At this time, since all the required information exist in a band between 44 MHz and 6 MHz of a digital broadcasting signal, the SAW filter 102 removes all the periods other than a band of 6 MHz with information from the output of the tuner 101 and outputs the signal of 6 MHz to the A/D converter 103.
The A/D converter 103 samples the output of the SAW filter 102 with a fixed frequency of 25 MHz and digitizes the sampled frequency. A matched filter 104 adjusts a signal-to-noise ratio (SNR) of the digitized signal at a symbol position and outputs the digitized signal to a resampler 105 for recovery of a digital symbol clock.
The resampler 105 receives a timing error of current symbols generated by signal processing of a base band from a timing recovery portion 109 and interpolates the timing error to reduce an error between the digitized signals. That is, the digital signal sampled at 25 MHz is interpolated at n times (n=2 in case of VSB) of a real symbol rate through the resampler 105.
The output of the resampler 105 is input to a phase divider 106, divided into components I and Q by the phase divider 106, and output to the multiplier 107.
The multiplier 107 receives a complex sinusoidal wave with a recovered carrier wave through a numerically controlled oscillator (NCO) 116 and multiplies the complex sinusoidal wave by signals I and Q of a pass band output from the phase divider 106 and lowers the signals I and Q of the pass band to a base band.
The signals I and Q of the base band output from the multiplier 107 are output to the timing recovery portion 109, the carrier wave recovery portion 113, and the channel equalizer 108.
At this time, once the output of the multiplier 107 passes through the channel equalizer 108, distortion generated by passing through a channel included in the output of the multiplier 107 is compensated. Also, the channel equalizer 108 recovers synchronizing signals inserted during transmission from the signal I of the base band and recovers received data, i.e., transmitting symbols using the synchronizing signals.
The carrier wave recovery portion 113 removes phase error and frequency offset of the carrier wave from a pilot signal of the base band output from the multiplier 107 and feeds back a corresponding complex sinusoidal wave to the multiplier 107. Therefore, the multiplier 107 outputs the digital signals I and Q of the base band with the recovered frequency offset and phase error.
To this end, the carrier wave recovery portion 113 includes a frequency phase error detector (FPED) 114, a loop filter 115, and an NCO 116. That is, the FPED 114 detects the frequency offset and the phase error from the pilot signal of the base band output from the multiplier 107 and outputs the detected frequency offset the phase error to the loop filter 115.
The loop filter 115 filters and integrates the output of the FPED 114 and outputs a final value to the NCO 116. The NCO 116 generates a complex sinusoidal wave relative to the output of the loop filter 115 and outputs the complex sinusoidal wave to the multiplier 107.
In other words, frequency offset of several hundreds of KHz and phase error are generated by a tuner or an RF oscillator during VSB signal reception. The frequency offset and phase error are required to be minimized for exact data recovery. At this time, acquisition and tracking are performed to minimize the frequency offset and phase error. This acquisition and tracking process is called carrier wave recovery.
Meanwhile, the timing recovery portion 109 generates the same symbol clock as that used during transmission to recover transmission data at a receiving party. This is because that an advanced television systems committee (ATSC) VSB transmission system proposed by a US directed digital TV (DTV) receiver loads data only in a transmission signal. At this time, the timing recovery portion 109 implements timing recovery during a data segment synchronizing signal period regularly inserted in a transmitting party.
To this end, the timing recovery portion 109 includes a timing error detector 110, a low pass filter (LPF) 111, and an NCO 112. The timing error detector 110 detects timing error information from the output of the multiplier 107. The LPF 111 passes through low band signal components only from the timing error information. The NCO 112 controls sampling timing of the resampler 105 by converting the output frequency in accordance with low band components of the timing error.
In other words, the timing error detector 110 detects the timing error information from the output of the multiplier 107 and outputs the detected timing error information to the LPF 111. The LPF 111 filters the low band signal components only from the timing error information detected by the timing error detector 110 and outputs the filtered low band signal components to the NCO 112. The NCO 112 converts the output frequency in accordance with the low band components of the timing error information and controls sampling timing of the resampler 105.
At this time, the carrier wave recovery portion 113 detects frequency phase error from a band with the pilot signal on the frequency spectrum. The timing recovery portion 109 detects timing error information from an opposite band with no pilot signal.
As described above, the VSB demodulating device of FIG. 1 is implemented by only a digital device without depending on an analog device. Accordingly, the VSB demodulating device can be implemented by an optimal parameter.
However, the VSB demodulating device of FIG. 1 is configured such that the carrier wave recovery portion 113 and the timing recovery portion 109 do not operate independently but operate to affect each other. In this case, once a problem occurs in the timing recovery portion 109, the problem directly affects the carrier wave recovery portion 113, and vice versa. As a result, receiving characteristic of the DTV receiver is degraded.
Furthermore, the carrier wave recovery portion 113 of FIG. 1 has an asymmetrical frequency offset acquisition performance. That is, the carrier wave recovery portion 113 of FIG. 1 has lower acquisition performance in case of the frequency offset in “−” direction than that in case of the frequency offset in “+” direction.
At this time, since the carrier wave recovery portion 113 and the timing recovery portion 109 operate to affect each other, degradation of acquisition performance in the carrier wave recovery portion 113 is linked to degradation of performance in the timing recovery portion 109.
Also, if a fatal ghost exists in a band used for timing recovery, a signal at a portion where the ghost exists is removed during demodulation. If the timing error is detected using such a signal, wrong timing error information may be detected. In this case, the carrier wave recovery portion 113 is affected by the timing error recovery portion 109, it is not operated normally. If the carrier wave is not recovered, the received data is not recovered normally, thereby degrading performance of the DTV receiver.